.\" Copyright (C) 2005, 2008, Michael Kerrisk <mtk.manpages@gmail.com>
.\" (A few fragments remain from an earlier (1992) version by
.\" Drew Eckhardt <drew@cs.colorado.edu>.)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2005, mtk: added an example program
.\" Modified 2008-01-09, mtk: rewrote DESCRIPTION; minor additions
.\"     to EXAMPLE text.
.\" 2008-10-10, mtk: add description of pipe2()
.\"
.TH pipe 2 2023-02-05 "Linux man-pages 6.03"
.SH NAME
pipe, pipe2 \- create pipe
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int pipe(int " pipefd [2]);
.PP
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.BR "#include <fcntl.h>" "              /* Definition of " O_* " constants */"
.B #include <unistd.h>
.PP
.BI "int pipe2(int " pipefd "[2], int " flags );
.PP
/* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
   following prototype; see NOTES */
.PP
.B #include <unistd.h>
.PP
.B struct fd_pair {
.B "    long fd[2];"
.B "};"
.B struct fd_pair pipe(void);
.fi
.SH DESCRIPTION
.BR pipe ()
creates a pipe, a unidirectional data channel that
can be used for interprocess communication.
The array
.I pipefd
is used to return two file descriptors referring to the ends of the pipe.
.I pipefd[0]
refers to the read end of the pipe.
.I pipefd[1]
refers to the write end of the pipe.
Data written to the write end of the pipe is buffered by the kernel
until it is read from the read end of the pipe.
For further details, see
.BR pipe (7).
.PP
If
.I flags
is 0, then
.BR pipe2 ()
is the same as
.BR pipe ().
The following values can be bitwise ORed in
.I flags
to obtain different behavior:
.TP
.B O_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the two new file descriptors.
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
.TP
.BR O_DIRECT " (since Linux 3.4)"
.\" commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d
Create a pipe that performs I/O in "packet" mode.
Each
.BR write (2)
to the pipe is dealt with as a separate packet, and
.BR read (2)s
from the pipe will read one packet at a time.
Note the following points:
.RS
.IP \[bu] 3
Writes of greater than
.B PIPE_BUF
bytes (see
.BR pipe (7))
will be split into multiple packets.
The constant
.B PIPE_BUF
is defined in
.IR <limits.h> .
.IP \[bu]
If a
.BR read (2)
specifies a buffer size that is smaller than the next packet,
then the requested number of bytes are read,
and the excess bytes in the packet are discarded.
Specifying a buffer size of
.B PIPE_BUF
will be sufficient to read the largest possible packets
(see the previous point).
.IP \[bu]
Zero-length packets are not supported.
(A
.BR read (2)
that specifies a buffer size of zero is a no-op, and returns 0.)
.RE
.IP
Older kernels that do not support this flag will indicate this via an
.B EINVAL
error.
.IP
Since Linux 4.5,
.\" commit 0dbf5f20652108106cb822ad7662c786baaa03ff
.\" FIXME . But, it is not possible to specify O_DIRECT when opening a FIFO
it is possible to change the
.B O_DIRECT
setting of a pipe file descriptor using
.BR fcntl (2).
.TP
.B O_NONBLOCK
Set the
.B O_NONBLOCK
file status flag on the open file descriptions
referred to by the new file descriptors.
Using this flag saves extra calls to
.BR fcntl (2)
to achieve the same result.
.TP
.B O_NOTIFICATION_PIPE
Since Linux 5.8,
.\" commit c73be61cede5882f9605a852414db559c0ebedfd
general notification mechanism is built on the top of the pipe where
kernel splices notification messages into pipes opened by user space.
The owner of the pipe has to tell the kernel which sources of events to watch
and filters can also be applied to select
which subevents should be placed into the pipe.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned,
.I errno
is set to indicate the error, and
.I pipefd
is left unchanged.
.PP
On Linux (and other systems),
.BR pipe ()
does not modify
.I pipefd
on failure.
A requirement standardizing this behavior was added in POSIX.1-2008 TC2.
.\" http://austingroupbugs.net/view.php?id=467
The Linux-specific
.BR pipe2 ()
system call
likewise does not modify
.I pipefd
on failure.
.SH ERRORS
.TP
.B EFAULT
.I pipefd
is not valid.
.TP
.B EINVAL
.RB ( pipe2 ())
Invalid value in
.IR flags .
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENFILE
The user hard limit on memory that can be allocated for pipes
has been reached and the caller is not privileged; see
.BR pipe (7).
.TP
.B ENOPKG
.RB ( pipe2 ())
.B O_NOTIFICATION_PIPE
was passed in
.I flags
and support for notifications
.RB ( CONFIG_WATCH_QUEUE )
is not compiled into the kernel.
.SH VERSIONS
.BR pipe2 ()
was added in Linux 2.6.27;
glibc support is available starting with glibc 2.9.
.SH STANDARDS
.BR pipe ():
POSIX.1-2001, POSIX.1-2008.
.PP
.BR pipe2 ()
is Linux-specific.
.SH NOTES
.\" See http://math-atlas.sourceforge.net/devel/assembly/64.psabi.1.33.ps.Z
.\" for example, section 3.2.1 "Registers and the Stack Frame".
The System V ABI on some architectures allows the use of more than one register
for returning multiple values; several architectures
(namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64)
(ab)use this feature in order to implement the
.BR pipe ()
system call in a functional manner:
the call doesn't take any arguments and returns
a pair of file descriptors as the return value on success.
The glibc
.BR pipe ()
wrapper function transparently deals with this.
See
.BR syscall (2)
for information regarding registers used for storing second file descriptor.
.SH EXAMPLES
.\" fork.2 refers to this example program.
The following program creates a pipe, and then
.BR fork (2)s
to create a child process;
the child inherits a duplicate set of file
descriptors that refer to the same pipe.
After the
.BR fork (2),
each process closes the file descriptors that it doesn't need for the pipe
(see
.BR pipe (7)).
The parent then writes the string contained in the program's
command-line argument to the pipe,
and the child reads this string a byte at a time from the pipe
and echoes it on standard output.
.SS Program source
.\" SRC BEGIN (pipe.c)
.EX
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    int    pipefd[2];
    char   buf;
    pid_t  cpid;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <string>\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (pipe(pipefd) == \-1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    cpid = fork();
    if (cpid == \-1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if (cpid == 0) {    /* Child reads from pipe */
        close(pipefd[1]);          /* Close unused write end */

        while (read(pipefd[0], &buf, 1) > 0)
            write(STDOUT_FILENO, &buf, 1);

        write(STDOUT_FILENO, "\en", 1);
        close(pipefd[0]);
        _exit(EXIT_SUCCESS);

    } else {            /* Parent writes argv[1] to pipe */
        close(pipefd[0]);          /* Close unused read end */
        write(pipefd[1], argv[1], strlen(argv[1]));
        close(pipefd[1]);          /* Reader will see EOF */
        wait(NULL);                /* Wait for child */
        exit(EXIT_SUCCESS);
    }
}
.EE
.\" SRC END
.SH SEE ALSO
.BR fork (2),
.BR read (2),
.BR socketpair (2),
.BR splice (2),
.BR tee (2),
.BR vmsplice (2),
.BR write (2),
.BR popen (3),
.BR pipe (7)
